1. Field of the Invention
The present invention relates to lithographic apparatus and, more particularly, to an improved interferometer system employed in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion or target field of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask (i.e., reticle), may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target field (e.g. comprising part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions or fields that are successively exposed. Known lithographic apparatus include so-called “steppers,” in which each target field is irradiated by exposing an entire pattern onto the target field in one sweep, and so-called “scanners,” in which each target field is irradiated by scanning the pattern through the projection beam in a given direction (e.g., the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion/field”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool.
Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
A support structure supports (i.e. bares the weight), of the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or substrate holders and/or two or more mask tables or mask holders. In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g. water), so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
Within a lithographic apparatus, interferometer systems may be used to measure distance and/or displacement. A basic interferometer system uses two, or more, bundles of radiation that are each reflected by a mirror, and are made to overlap and interfere. The overlap may cause constructive or destructive interference, depending on the phase difference of the radiation in the two bundles. This is visible in the form of interference fringes.
Because the wavelengths of each bundle are generally equal, a single bundle is typically split in two bundles by means of a beam-splitter. Another possibility is the use of a heterodyne system, in which distinct but very nearly equal frequencies are split up by the beam-splitter. This often relates to two Zeeman split frequencies having perpendicular polarization directions. The beam-splitter will be a polarizing beam-splitter. This offers the possibility of redirecting bundles by means of the beam-splitter surface before and/or after reflection.
A particular and well known advantage of interferometer systems is that it is possible to measure extremely small displacements, in the order of fractions of a wavelength. In the simplest form of the interferometer system, one interference fringe appears every half wavelength. However, it is possible to feed the interference signal to electronic detectors which are able to interpolate the measured signal in order to determine smaller displacements and increase measurement resolution.
Some lithographic systems employ a combination of interferometers that include a first interferometer system to measure displacement in substantially the X direction and a second interferometer system to measure displacement in the Z direction. In this combination, the measuring and reference beams of the second interferometer system (i.e., the Z measurement) are formed in a separate beam-splitter and are guided through bores in the second beam-splitter, which forms the reference beam and measuring beam of the first interferometer system (i.e., the X measurement). Note that the beam generation means comprise two separate beam-splitters, each of which split radiation beams in respective measuring beams and reference beams.
A disadvantage of the combination interferometer system is the complexity of the overall system, requiring two beam-splitters and the drilling of holes in one of them. Because beam-splitters are one of the most expensive parts of an interferometer system, the cost of such a system may be lowered and improved by providing a configuration that reduces the cost for the beam-splitters. Furthermore, the presence of holes for guiding the beams for the Z interferometer system limits the flexibility of the combined system and requires two labor-intensive alignment procedures.